The use of a cluster ion beam for processing surfaces is known (see for example, U.S. Pat. No. 5,814,194, Deguchi et al.) in the art. As the term is used herein, gas-clusters refer to nano-sized aggregates of materials that are gaseous under conditions of standard temperature and pressure. Such gas-clusters are typically comprised of aggregates of from a few to several thousand molecules loosely bound to form the cluster. The clusters can be ionized by electron bombardment or other means, permitting them to be formed into directed beams of controllable energy. Such ions each typically carry positive charges of q·e (where e is the electronic charge and q is an integer of from one to several representing the charge state of the cluster ion). Non-ionized clusters may also exist within a cluster ion beam. The larger sized cluster ions are often the most useful because of their ability to carry substantial energy per cluster ion, while yet having only modest energy per molecule. The clusters disintegrate on impact, with each individual molecule carrying only a small fraction of the total cluster ion energy. Consequently, the impact effects of large cluster ions are substantial, but are limited to a very shallow surface region. This makes cluster ions effective for a variety of surface modification processes, without the tendency to produce deeper subsurface damage characteristic of conventional monomer ion beam processing.
Means for creation of and acceleration of such GCIBs are described in the reference (U.S. Pat. No. 5,814,194) previously cited, the contents of which are incorporated by reference as though set out at length herein. Presently available cluster ion sources produce clusters ions having a wide distribution of sizes, N (where N=the number of molecules in each cluster ion—in the case of monatomic gases like argon, an atom of the monatomic gas will be referred to as a molecule and an ionized atom of such a monatomic gas will be referred to herein as a molecular ion or simply a monomer ion.)
Many useful surface processing effects can be achieved by bombarding surfaces with GCIBs. These processing effects include, but are not necessarily limited to, cleaning, smoothing, etching, doping, and film formation or growth. In many cases, it is found that in order to achieve industrially practical throughputs with GCIB processing, GCIB currents on the order of hundreds to thousands of microamps are required. Efforts to increase the intensity and ionization of a GCIB beam tend to produce additional higher charge state clusters (q>1). When ionization is performed by electron bombardment, ionization is produced by random electron impacts. In order to produce a high ratio of ionized to non-ionized clusters, the electron impact probability must be high and the resulting charge state distribution follows approximately Poisson statistics, with the approximate probability, P(q), of charge state q given by:
                              P          ⁡                      (            q            )                          =                                                            q                _                            q                                      q              !                                ⁢                      ⅇ                          -                              q                _                                              ⁢                      i            .                                              (                  Eqn          .                                          ⁢          1                )            where {overscore (q)} is the average ionized cluster charge state after leaving the ionizer. Thus, an ionized cluster beam with a highly ionized fraction will also include multiply-charged cluster ions in the beam. For example, theoretically the average cluster charge state of a GCIB beam where 95% of the clusters are ionized would be 3, with more than 8% of the beam in charge states 6 and higher. However, such highly charged clusters can fragment, or undergo charge exchange reactions, or partially evaporate, resulting in a different charge state distribution and/or a different energy distribution, and so in a practical beam, the precise charge state and energy distributions are not readily predicted.
In the prior art, it has been understood that optimal ion beam propagation is generally achieved under low-pressure conditions. It has also been understood that the moderate to high intensity GCIBs, as are normally required for efficient surface processing of materials on an industrially economic scale, transport substantial quantities of gas in the form of gas-cluster ions to the target region. When gas-cluster ions in a GCIB reach the target, the clusters disintegrate and the GCIB-transported mass is released as molecular gas. The entire gas load of the beam is released when the GCIB strikes the target region. For an argon beam having a beam current, IB, the gas flow, F (sccm), transmitted in the beam is:
                    F        =                  2.23          ×                      10                          -              18                                ⁢                      (                          N              q                        )                    ⁢                      (                                          I                B                            e                        )                                              (                  Eqn          .                                          ⁢          2                )            
Accordingly, for a beam current of only 400 μA and an N/q ratio of 5000, the beam transmits a substantial gas flow of about 27 sccm. In a typical GCIB processing tool, multiple large capacity vacuum pumps are employed in order to maintain a low-pressure environment in the face of this gas load.
Because high intensity GCIBs contain clusters of various charge states, acceleration of such beams by applying an accelerating potential, VAcc, of a few kV results in beams having clusters of multiple energies, q·VAcc for all values of q present in the beam. In general, it has been learned that many of the beneficial processing effects that can be obtained by the use of GCIB for processing workpiece surfaces are dependent on the energies of the gas-cluster ions in the beam. Etching of a surface, for example, generally proceeds faster by using higher energy clusters. Another valuable application of GCIB processing is the smoothing of surfaces, and GCIB has been shown to be in some cases superior to other methods for smoothing surfaces at the atomic or near-atomic scale. Although for some beam conditions GCIB processing of a surface can produce exceptional smoothness, it has been observed that GCIB processing does not always smooth a surface. In fact, when the starting surface is relatively smooth, GCIB processing may, in some circumstances, roughen the surface. Often it is desirable to both etch and smooth a surface. When using conventional techniques to optimize processing conditions (for example, by selecting cluster source gas material, by selecting acceleration potential, by selecting a GCIB current, and/or by selecting a GCIB processing dose) it has been found that frequently there is not a GCIB beam condition that gives an adequate etching rate while simultaneously smoothing, or at least not roughening, an already smooth surface. It has been found that an aggressive etch rate by GCIB processing commonly requires high energy, high intensity GCIB conditions, while freedom from roughening and surface smoothing are best obtained with low energy beams (or with conditions that are otherwise not practical for etching). In such cases it has been necessary to use a combination of several GCIB steps to achieve a result approaching a desired objective. Such a process would involve first an aggressive etch with a first set of beam conditions, followed by a second less aggressive etch using a second set of beam conditions to reduce the roughening caused by the first etch, and then yet another step that applies a beam condition that smoothes without any significant etching. Such combinations of steps or even more complex combinations are known in the art and result in complex processing recipes with low throughputs for certain important processes, while sometimes still not achieving desired final results. U.S. Pat. No. 6,375,790 to Fenner, for example, teaches a GCIB processing apparatus adapted for multi-step processing of substrates capable of such complex processing recipes.
It is the case that the available ionizers for producing ionized gas-clusters for GCIB formation inherently produce beams with a wide range of ionization states, and particularly including multi-ionized gas-clusters when operated at conditions that produce intense beams (see Eqn. 1 above) needed for high throughput GCIB processing of workpieces. When such beams are accelerated with potentials adequate to give energies that provide good etching rates, they tend to produce less-than-desirable surface smoothing and may even produce surface roughening. This problem can be at least partially alleviated by complex or multi-step GCIB processing recipes that reduce processing throughput below desirable rates. However, sometimes even complex processing recipes with several different beam conditions do not produce the desired ultimate level smoothness needed for some important processes.